1. Field of the Invention
The present invention relates to a variable focal length optical system that is suitable for photographing lenses used for cameras.
2. Related Background Art
Recently, variable focal length optical systems are going mainstream as photographing lenses for lens shutter-type cameras. With sophisticated variable focal length optical systems, various types of variable focal length optical systems have been proposed. In recent years, the advance of technology on lens barrels allows proposing various variable focal length optical systems with great zoom ratio that use so called multiple variable focal length optical systems each of which is formed of at least three movable groups. Various proposals regarding the multiple variable focal length optical system focusing method have been made with such an advanced technology.
Generally, the so called multiple variable focal length optical system can provide the degrees of freedom of zooming locus of a movable lens group when a refractive power is varied from a wide-angle end to a telephoto end. Hence, when the aberration correction is carried out, the freedom of lens designing increases, and changing air gaps for zooming increase, so the share of zooming operation to each lens group is reduced. Moreover, the advance of the lens barrel technology has overcome somewhat the problem that the lens barrel structure is complicated with movable portions increased, thus increasing the degrees of freedom in optical engineering. As a result, it is possible to design higher performance variable focal length optical systems with high variable refractive power.
In conventional variable focal length optical systems with no limitation in back focus, in order to reduce the entire length of a lens system and to reduce the diameter of each lens, a negative lens group is arranged at a position nearest to an image. A refractive power adjustment is effectively made ranging from a wide-angle end to a telephoto end by varying largely the back focus. A positive lens group is arranged at a position nearest to an object in a lens system. It is engineered to reduce the entire length of the lens system by arranging the positive lens group at a telephoto end near the object, in comparison with its position at a wide-angle end.
As described above, a three-group variable focal length optical system including a positive lens group, a positive lens group and a negative lens group, or a four-group variable focal length optical system including a positive lens group, a negative lens group, a positive lens group and a negative lens group is concretely listed as a multiple variable focal length optical system which is suitable for its high variable refractive power operation and its compactness. Conventionally various proposals have been made for such a lens structure.
Japanese Laid-open Patent Application No. 2-256015, for example, discloses a positive, positive, and negative-type three-group variable focal length optical system. This variable focal length optical system includes a first lens group with a positive refractive power, a second lens group with a positive refractive power with an air gap therebetween maintained to be unchanged which is formed of a second lens group a, and a second lens group b, and a third lens group with a negative refractive power, the first, second and third lens groups being arranged in order from the side of an object. With a variable refractive power operation ranging from a wide-angle end to a telephoto end, the air gap between the second lens group and the third lens group is decreased while the air gap between the first lens group and the second lens group is increased.
In addition, Japanese Laid-open Patent Application No. 60-57814 discloses a positive, negative, positive, and negative-type four-group variable focal length optical system. This variable focal length optical system includes a first lens group with a positive refractive power, a second lens group with a negative refractive power, a third lens group with a positive refractive power, and a fourth lens group with a negative refractive power which are arranged in order from the side of an object. With a variable refractive power operation ranging from a wide-angle end to a telephoto end, the air gap between the first lens group and the second lens group is decreased while the air gap between the third lens group and the fourth lens group is increased. In order to reduce the entire lens length at a telephoto end, the refractive power of the first lens group is positively stronger than that of the entire lens system at a telephoto end and the composite refractive power of the second lens group and the fourth lens group at the telephoto end is negative.
However, it is difficult for the conventional multiple variable focal length optical systems to achieve a good imaging performance with a small number of lenses, thus maintaining a high variable refractive power, and compactness in size. For example, in the variable focal length optical system disclosed on the Japanese Laid-open Patent Application No. 2-256015, in the case of a high variable refractive power operation, the third lens group, which is burdened with most of the variable refractive power operation causes a large variation in off-axis aberration is a variable refractive power operation ranging from a wide-angle end to a telephoto end. In order to suppress a variation in aberration due to the variable refractive power operation, the number of lenses forming the second lens group may be increased, or the air gap along an axis between the second lens group a and the second lens group b may be sufficiently widened. However, such an approach defies the object of the present invention which is a reduced number of lenses and compactness in a variable focal length optical system.
Moreover, according to Japanese Laid-open Patent Application No. 60-57814, 1/.beta.2.congruent.0, where .beta.2 is a use refractive power (magnification) of the second lens group at a wide-angle end. In this case, since the composite refractive power of each of the first and second lens groups is nearly 0, a design for a high-angle causes an insufficient back focus, thus increasing the effective diameter of the rear lens.
In the lens described above, the first to third lens groups are burdened with the variable refractive power operation ranging from a wide-angle end to a telephoto end by varying largely the air gap between the second lens group and the third lens group. Thus the lens structure allows a higher variable refractive power operation, compared with the positive, positive and negative-type three-group variable focal length optical system. However, in the case while a high variable refractive power operation as well as compactness are desired, it is difficult to perform good correction of a variation in an off-axis aberration occurring in the second lens group because an off-axis luminous flux does not virtually vary its height passing the second lens group but varies only its incident angle.
As described above, it is difficult to establish high performance in addition to compactness and high variable refractive power operation using the conventional positive, positive, and negative-type three-group variable focal length optical system or a positive, negative, positive, and negative-type four-group variable focal length optical system. Realizing a higher variable refractive power operation using a small number of lenses increases a burden of each lens group at the time of variable refractive power operation. Hence, it is difficult to correct properly a variation in aberrations in a variable refractive power operation ranging from a wide-angle to a telephoto end.
Next, let us explain focusing of the multiple variable focal length optical system.
Generally, it is required that a lens group, or focusing group, that moves along the optical axis to focus a variable focal length optical system to an object has small movement and small weight. This approach has an advantage in that reducing the movement of the focusing group leads to the compactness of an entire lens system. Making the focusing group light of weight has the advantage of simplifying the mechanism of driving it.
Conventionally, in order to focus the multiple variable focal length optical system to an object positioned from a long distance to a short distance, there are three focusing methods:
(A) FF (Front Focusing) method
(B) IF (Inner Focusing) method
(C) RF (Rear Focusing) method.
Various proposals are advanced for the methods above.
In the multiple variable focal length optical system which has no limitation for back focus, including the conventional positive, positive, and negative-type three-group variable focal length optical system or the conventional positive, negative, positive, and negative-type four-group variable focal length optical system, a positive lens group is arranged at a position nearest to an object in a lens system. However, in order to provide good correction to a distortion aberration occurring at a wide-angle end, the refractive power of the positive lens group at a position nearest to the object is adjusted slightly to a positive side. For that reason, the focusing according to the front focusing method (A) causes an excessive movement of the first lens group, which is a focusing group.
A negative lens group is arranged at a position nearest to an image, and the back focus is short at a wide-angle end. Therefore, the negative lens group which has a lens diameter larger than the lens diameter of another lens group is largely burdened with the variable refractive power of the negative lens group. For that reason, at least two lenses are required to form a negative lens group at a position nearest to an image. The rear focusing method (C) is not preferable to perform a focusing operation because the negative lens group which is a focusing group, gains weight.
In the case of the use of the inner focusing method (B), as disclosed in, for example, Japanese Laid-open Patent Application No. 4-338910, the second lens group is divided into a front subgroup and a rear subgroup to move the front subgroup of the second lens group toward an object, thus performing a focusing operation. However, using the front subgroup of the second lens group as a focusing group, which is formed of a large number of lenses and has a weak positive refractive power, results in an excessive movement of the focusing group at a short distance focusing operation.
On the other hand, the positive, negative, positive, and negative-type variable focal length optical system, as disclosed in Japanese Laid-open Patent Application No. 3-39920, executes a focusing operation using the second lens group. However, since the second lens group varies largely in its use refractive power during a refractive power varying operation, the number of lenses forming the second lens group has to be increased to suppress a variation in aberration during a focusing operation together with a variation in aberration occurring at a refractive power varying time. As a result, using the second lens group as a focusing group results in undesirably increased weight.
In the variable focal length optical system disclosed in Japanese Laid-open Patent Application No. 3-225309, a focusing operation is performed by moving plural lens groups. However, unlike telephoto variable focal length optical systems, a wide-angle variable focal length optical system needs to correct properly the off-axis aberration occurring during at a focusing operation over wider angles. Therefore, the variable focal length optical system of the type disclosed in Japanese Laid-open Patent Application No. 3-225309 cannot provide a sufficient correction against a variation in off-axis aberration.
As described above, in the conventional multiple variable focal length optical systems, it has been difficult to provide good imaging performance with a small number of lenses, in addition to a realization of a wider angle, a high variable refractive power, and a reduced size. Moreover, it is not sufficient to reduce work (=weight.times.movement) during a focusing operation.